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Winged perfume producers

Winged perfume producers

How orchid bees find their personal scent and attract mates

by
Julia Weiler

May 6, 2014

A droplet from an orchid flower, the woody scent of tree bark and a dash of fungus aroma: orchid bees are passionate scent collectors. In the course of their lives, the males compile a species-specific bouquet that they store in pockets on their hind legs – in order to attract females, assumes Tamara Pokorny from the Department of Animal Ecology, Evolution and Biodiversity. She researches the flying perfume aficionados' collecting behaviour.

Orchid bees owe their German name (“Prachtbiene”, which loosely translates to “splendorous bee”) to their colourful surface; but an even more characteristic feature is their affinity for fragrances, which was first discovered in connection with fragrant orchids. "The males spend all their life flying around and collecting scents," says Tamara Pokorny (fig. 1). Members of different species are interested in different scents. Orchid flowers are among the most popular sources, but eucalyptus-scented eucalyptol is also frequently added to the blend. However, it is not just substances that have a pleasant smell to humans that find their way into the male bees' pockets; some species like to include skatole, a compound occurring in faeces, as part of the bouquet.

A male bee's complete perfume is made up of 20 to 40 characteristic components on average. Generally, the bouquets of young individuals deviate considerably from the ideal blend, because they had not yet had the chance to collect as many scents as older bees. This is evident in the data gathered by Tamara Pokorny and Marko Hannibal during their work as visiting researchers in Mexico (fig. 2). The smaller the content of an orchid bee's collecting pocket, the further away it is from its species' ideal bouquet. The biologist from Bochum also found out that bees are aware of what they had already collected. Even if an attractive scent is available in abundance, the bees stop picking it up at some point. "They remember: 'I've got this already,' and they scout for something else," explains Tamara Pokorny.

At some point in their life – and it is not yet known what the trigger is – the males decide to use their bouquet to mark and establish their territory (video). They select a tree trunk where they perch and fly around it; by moving their legs in a specific way, they disperse the scent from their collecting pockets through the air. "Thus, they attract individuals of their own species," says Tamara Pokorny. It is yet unclear if it is females looking for a mate that are attracted or males competing with their rivals for the territory in a 'fitness contest'.

What is clear is that the selection of the tree trunk that becomes the centre of the territory does not happen randomly. Together with a team of students, Tamara Pokorny analysed several orchid bee species and their favourite trees in Costa Rica. To this end, she first had to track down the bees in the rain forest. "I move very quietly through the forest, pause and listen for the bees. With a bit of practice, it is possible to discern them; they have a very distinct buzz," says the PhD student. Once she catches the sound, she looks around until she spots the bee. "Most of the time, we see one of four species that can be easily identified optically." This is how Tamara Pokorny observed that orchid bees prefer landing on smooth rather than rough tree trunks and that different species alight on the trunks at different heights from the ground. The smaller species prefer branches or trunks with a smaller diameter, larger species look for trunks with a larger diameter. It is yet to be determined if the type of tree is in any way decisive. "In the tropics, many types of trees are very similar to each other; it is difficult to identify them," explains Tamara Pokorny. "But it looks like each species prefers a certain type of tree."

In the next step, she plans to determine the extent to which the choice is affected by the direction of the wind. The biologist ventures a prognosis even before the final analysis of the collected data: "It looks like the bee moves over to the other side of the trunk when the wind changes." When insects want to locate the source of a scent, they generally approach it by flying against the air current. Apparently, male orchid bees place themselves on the trunk into the wind so that any incomers spot them when flying towards the scent.

Orchid bees are generally good flyers; this is yet another aspect that Tamara Pokorny looked into in her PhD thesis. On the Yucatán Peninsula in Mexico, she subjected the bees to a marathon test. The biologist lured them in with scents (fig. 3), marked them and let them fly over the monotonous plain. She set up further bait stations in various distances and checked which of the marked bees made it that far. "Orchid bees can fly as far as 50 kilometres," she concludes. "An amazing feat for an animal that is barely twelve millimetres long!"

This posed the question from where the insects derived the energy to fly such long distances. Generally, orchid bees replenish their energy by taking in sugar through nectar. Due to their extremely long proboscis, they can only drink thin sugar mixtures (fig. 4). "This is as if a human were to drink through a very long sangria straw. The thicker the drink, the more difficult it becomes," explains Tamara Pokorny. Accordingly, orchid bees find sucking in liquids with lower sugar content easier; however, if they want to fly long distances without having to pause to take in nutrients from flowers, they require large sugar reserves. This poses a problem, because the small bees' stomachs do not offer a lot of space. Tamara Pokorny assumes that the bees concentrate the sugar solution. They evaporate the superfluous water, retaining only the energy source within their body. After drinking, the bees move their proboscises in a specific manner – a behaviour that may be necessary for concentrating the absorbed liquid.

The biologist conducted an experiment to test this theory. She caught several bees and accustomed them to drinking from artificial flowers with different sugar contents. After noting the time the bees spent with the proboscis movement, she made the bees regurgitate by carefully applying pressure to their abdomen in order to analyze the contents of their stomachs. Thus, she could compare the original sugar content in the flower with the sugar content in the stomach. The longer the bees had moved their proboscises after drinking, the higher the sugar content in their stomach, compared to the original volume. Apparently, the proboscis movement is indeed used to concentrate the sugar content.

Tamara Pokorny has already found out a lot about the behaviour of orchid bees, but there is still a lot left to explore. In the next step, she plans to ascertain if any other scents have also significance as perfumes, in addition to those in the hind leg pockets. The bodies of many insects are covered in a lipid layer that protects them from evaporation and, at the same time, contains scents that help individuals recognise members of the same species. The question is why do orchid bees need a second scent system? "It's possible that there is a difference between long-distance and short-distance attraction," assumes Tamara Pokorny. The scents that make up the individually compiled perfume are volatile and can be smelled over long distances; the fat layer may be useful to recognise members of the same species in the immediate vicinity. "It would be most inconvenient if a female took a bouquet to mean that her noble knight is near, only to end up joining a male of the wrong species." It remains to be seen if the biologist's theory will prove right in the long term.

Orchid bees and orchids

More than 200 orchid bee species live in the neotropics of Central America and the northern parts of South America. Even though they are closely related to honey bees, they have a completely different way of life; they are, for example, solitary and do not build large nests. Orchid bees grow to a size of eight to 28 millimetres and reach an average age of three months. They play an important role for the ecosystem in the tropics, as more than 500 types of orchids rely on being pollinated by orchid bees.

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